1. Field of the Invention
The field of this invention is that of flow rate and impact modifiers for polypropylene resin compositions.
2. Prior Art
A well known approach to increasing impact properties of polypropylene resins is to add ethylene-propylene copolymer rubbers and diene modifications thereof. Unfortunately, conventional additives when incorporated into polypropylene compositions while increasing impact resistance decrease the flow rate. An example of increased impact resistance coupled with decreased flow rates is shown in Table I.
"Impact resistance" is herein used to mean either the Izod impact strength as measured by ASTM D-256 wherein a sample is either notched or unnotched and tested at room temperature, or the Gardner impact test which involves a value determined from dropping a weight measured in pounds from a height measured in inches so as to just crack a 2 .times. 3 .times. 0.1 inch injection molded plaque which is at a temperature of -20.degree. F. The Gardner impact test, accurate to plus or minus 10%, is described in Paint Testing Manual edited by G. G. Sward and published as the ASTM Special Technical Publication 500, 13th Edition, page 335 (1972).
Flow rates as used throughout this specification and claims are defined to mean the number of grams flowing in 10 minutes as determined by ASTM D1238 Condition L.
The materials used in TABLE I are as follows:
Polypropylene 1001 which is commercially available from Amoco Chemicals Corporation having a flow rate of approximately 0.5-0.9 grams/ 10 minutes;
Polypropylene 1004 which is commercially available from Amoco Chemicals Corporation having a flow rate of approximately 2.4-4.0 grams/ 10 minutes;
Polypropylene 1008 which is commercially available from Amoco Chemicals Corporation having a flow rate of approximately 7-11 grams/ 10 minutes;
ECD 3252 which was obtained from E. I. du Pont de Nemours is a propylene/diene/ethylene terpolymer composed of 25% bound propylene, 2.7% bound 1,4-hexadiene, and 72.3% bound ethylene; and
Metrotalc 4615 which is a commercially available talc obtainable from Whittaker, Clark and Daniels.
TABLE 1 __________________________________________________________________________ Type of Polypropylene Used in a composition GARDNER IMPACT Modified with 20% FLOW RATE* (-20.degree. F) ECD 3252 and 10% with modifiers NOTCHED IZOD with modifiers Metrotalc 4615 (without modifiers) (Room Temp.) (without modifiers) __________________________________________________________________________ 1001 0.54 (.7) 9.50 120 (0-2) 1004 2.8 (5.4) 2.20 94-96 (0-2) 1008 6.1 (8.0) 1.03 76 (0-2) __________________________________________________________________________ *All flow rates were measured except for 1001 and 1008 (without modifiers). These exceptions are typical values.
Flow rates are directly related to the ease of which a mold cavity can be filled. A range of flow rates typically used in molding is 6-8 grams/10 minutes. Unfortunately, often no workable trade-off in flow rate and impact resistance can be found that will permit use of polypropylene having a flow rate near 6. This is often the case even though less modifier is required for lower flow rate polypropylene than higher flow rate polypropylene to achieve a significant increase in impact resistance as measured by the Gardner impact method at -20.degree. F.
It is an object of this invention to utilize in polypropylene compositions an impact modifier additive which will increase both impact resitance at room temperature and flow rate without significantly lessening flexural modulus or heat deflection temperatures.
Other objects are obvious to those skilled in the art based on the teachings disclosed herein.